The most widely used filters currently being employed in the manufacture of filter cigarettes are those prepared from a large number of continuous, longitudinally aligned filaments which have been combined and shaped into cylindrical form. The filaments generally used are prepared from cellulose acetate and they are supplied to the filter manufacturer in the form of a compacted bale of filter tow. In order to obtain filters that are as uniform as possible, the filter tow is subjected to various processing steps including steps for separating or spreading out the tow and applying plasticizer thereto.
Two basic methods for applying plasticizer to filter tow are widely used in the industry at the present time. The first method involves passing the filter tow through a chamber provided with spraying devices such as nozzles and centrifugal-type applicators. Apparatus utilizing this method is disclosed, for example, in U.S. Pat. Nos. 4,132,189 and 4,313,974 as well as in U.K. Patent Application Nos. 2,042,375 and 2,054,342. The second basic method of applying plasticizer involves contacting the tow with a surface that is coated with the desired quantity of plasticizer. This second method is exemplified by apparatus disclosed in U.S. Pat. Nos. 3,157,536, 3,306,254 and 3,411,942. For each of these basic methods the filter tow is spread out into a flat band or web configuration before the plasticizer is applied.
In previously disclosed apparatus such as that described in U.S. Pat. No. 3,157,536, the flat band of filter tow is contacted on each side thereof with wicktype applicators which are provided with stainless steel wire mesh surfaces across which the band of tow is moved. Uniform distribution of liquid plasticizer in this apparatus design is achieved by positioning a porous felt layer beneath the stainless steel wire mesh surface. After a period of time, however, deposits tend to build up in the porous felt layer leading to channeling and non-uniform application of plasticizer to the wire mesh surface and the band of filter tow. The subsequent passage of the treated tow through a blooming jet fails to compensate fully for this non-uniform application of plasticizer and the resulting filter rod formed from the tow is of correspondingly lower quality. It is apparent, therefore, that the plasticizer must be distributed on the filter tow as uniformly as possible prior to passage of the tow through the blooming jet. Consequently, the use of applicators such as those disclosed in U.S. Pat. No. 3,157,536 requires frequent cleaning if satisfactory operation is to be obtained.